This invention relates to a calcium-phosphate type hydroxyapatite and a process for producing the same.
More particularly, the invention is concerned with a novel calcium-phosphate type hydroxyapatite having specific crystal properties and useful as a column packing material for separation of biopolymers, as a material for biomedical use such as bone regeneration, and as a bioceramic material, catalyst support, fluorescent material, electronic material, inorganic ion exchanger, and genetic engineering material. The invention also concerns a process for producing the above hydroxyapatite.
The term apatite-structure compounds is generically used to denote a large group of compounds having a common formula M.sub.10 (ZO.sub.4).sub.6.X.sub.2 where M represents a metallic atom of Ca, Ba, Sn, Mg, Na, K, Pb, Cd, Zn, Ni, Fe, Al or the like, ZO.sub.4 represents an acid radical, such as PO.sub.4, AsO.sub.4, VO.sub.4, SO.sub.4, or SiO.sub.4, and X represents an OH or F atom (group). The present invention is directed to a calcium phosphate compound of the above general formula in which M is essentially Ca, ZO.sub.4 essentially PO.sub.4, and X essentially OH. The following description will, therefore, be centered around this particular compound.
The calcium-phosphate type hydroxyapatite, Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2 (hereinafter termed "hydroxyapatite" unless otherwise noted), has attracted growing attraction in recent years as a material for biomedical use, typically for artificial teeth and bones. This is because it closely resembles in composition to the inorganic ingredients of the teeth and bones of vertebrate animals and exhibits good affinity for living bodies. The affinity for living organisms has rendered it useful as a packing material to be filled in columns for chromatographic separation of biopolymers, for example, proteins, enzymes, and nucleic acids.
Conventionally, such hydroxyapatites are synthesized in most cases by
(1) wet synthesis involving an aqueous solution reaction in which a water-soluble calcium salt and phosphate are allowed to react in aqueous solution,
(2) dry synthesis involving a high-temperature solid-phase reaction in which calcium phosphate and calcium carbonate are allowed to react in the presence of water vapor at 900.degree. to 1400.degree. C., or
(3) hydrothermal synthesis involving a reaction under high-temperature, high-pressure steam in which calcium hydrogenphosphate is hydrolyzed, for example, at 200.degree. C. and at 15 atm.
In addition to these, a new synthetic process has been proposed, for example, by Patent Application Publication No. 500447/1980.
The hydroxyapatites obtained in the foregoing processes have a crystal structure belonging to the hexagonal system, with a space group of P6.sub.3 /m. Their unit lattice constants are 9.432 .ANG. for the a axis and 6.881 .ANG. for the c axis. [M. I. Kay & R. A. Young: Nature, 204, p. 1050 (Dec. 12, 1964)].
We have found, as a result of extensive studies and experiments, that the hydroxyapatites having the crystal properties as above are not capable of constantly achieving adequate separability and selectivity when used as column packing materials for chromatographic separation, especially, of biopolymers.
Therefore, it is an object of the present invention to provide a novel hydroxyapatite having unique crystal properties, or specific unit lattice constants, and useful as a material for biomedical applications as artificial teeth and bones, as a bioceramic material, catalyst support, fluorescent material, electronic material, inorganic ion exchanger, genetic engineering material and so forth, and also a process for producing the same.
Another object of the invention is, in view of the fact that the hydroxyapatites proposed thus far have had inadequate separability and selectivity when used as column packing materials for chromatographic separation of biopolymers, to provide a novel calcium-phosphate type apatite for chromatographic separation having unique crystal properties and excels in separability and selectivity.